Change LAP generator to use new PC-relative address cacheing

Change LAP generator to use new PC-relative address cacheing
mechanism.  Also eliminate generation of PC-RELATIVE-OFFSET pseudo
instruction, and of certain instructions that would make analysis of
the LAP difficult.  Change LAP to conform more closely to the MIPS
spec, e.g. change "FADD DOUBLE" to "ADD.D".  Implement simple LAP
optimizer that eliminates some NOPs appearing in load delay slots.